Hitherto, various types of power transmission devices for use in construction vehicles, agricultural machinery, automobiles, etc., such as of mechanical type, hydraulic type, and electric type, have been proposed and used. Regarding small vehicles for construction equipment, those of hydraulic type have been used relatively often. This is because those of hydraulic type can change their running speed from zero to infinity, and a merit of excellent operability has been highly regarded. On the other hand, those of hydraulic type have disadvantages of lower efficiency and higher cost as compared to those of mechanical type. However, operating machines for digging, earth moving, etc., are mounted in construction equipment such as a wheel hydraulic excavator, and all power from an engine is converted by a hydraulic pump in order to actuate the operating machine, so that the use of hydraulic driving apparatuses may becomes less expensive conversely.
In addition, the above-described hydraulic driving apparatuses include two types: the closed circuit and the open circuit. Since they have different characteristics, they are selected for use in accordance with the purpose. For example, in construction equipment, which is used mainly for the purpose of traveling when large amounts of flowing oil pressure are required, those of the open circuit type are used. Recently, closed center load sensing circuits have been used in operating machines in terms of improvement in operability, and closed center valves are adopted therein. Incidentally, when traveling efficiency or controllability is emphasized, those of closed circuit type are used.
Further, a prior art using a counterbalance valve comprises, as shown in FIG. 14, a variable displacement hydraulic pump 210 driven by a driving source 1 such as an engine; a capacity control device 211 for controlling the capacity of the hydraulic pump 210; a forward-reverse directional control valve 212; solenoid operated proportional valve 213 (forward) and 214 (reverse) for controlling the directional control valve 212; a counterbalance valve 215; a variable displacement hydraulic motor 216; and a capacity control device 217 for controlling the capacity of the hydraulic motor 216. This arrangement controls return oil from the counterbalance valve 215 to effect speed control (runaway prevention) when descending a slope.
However, such a prior art encounters the following problems. That is, the use of the counterbalance valve in a traveling circuit reduces efficiency because the forward-reverse directional control valve and the counterbalance are controlled by restriction. Moreover, since heat is generated while traveling, a larger radiator and a larger output engine are required, so that the size and cost of the vehicle increases.
In addition, the use of the closed center valve, in the traveling circuit of a vehicle emphasizing operability, encounters a similar malfunction because the valve is controlled by restriction similar to the counterbalance valve. Particularly, in a high-speed, long-distance traveling vehicle, the resistance increases, reducing efficiency, and the heating value also increases, so that a large radiator is required.
Further, the use of the open circuit type in a speed changing device for hydraulic driving apparatuses, such as construction vehicles emphasizing traveling efficiency and controllability, encounter the following problem. When a forward-reverse speed change, such as from neutral to forward, from forward to reverse and from reverse to forward, is effected, a start or a speed change operation is performed regardless of the present vehicle speed. During operation, if the shift lever does not correspond to the actual traveling direction of the vehicle, braking action is effected until the motor rotation is reduced to zero. When control is exercised without recognizing the braking action, there occurs a malfunction such that cavitation occurs and excessive braking is applied. For example, the case of switching to the reverse direction while rotating in the forward direction at high speed is shown in FIGS. 15A to 15F.
FIG. 15A shows pilot pressure for operating a directional control valve in the forward direction by actuation of a forward side solenoid operated proportional valve, and FIG. 15B shows a pilot pressure for operating the directional control valve in the reverse direction by actuation of a reverse side solenoid operated proportional valve, respectively. FIG. 15C shows a pressure Pa for rotating the hydraulic motor in the forward direction, and FIG. 15D shows a pressure Pb for rotating the hydraulic motor in the reverse direction, respectively. In addition, FIG. 15E shows a pilot pressure of a solenoid operated proportional valve for actuating an inclined shaft which controls displacement of the hydraulic motor. Further, FIG. 15F shows the rotational speed of the hydraulic motor output shaft in which the forward rotation output shaft decelerates.
An operation due to the switching will be described. Up to point W in FIG. 15A, the pilot pressure is applied in the forward direction, and the pilot pressure is applied in the reverse direction after point W in FIG. 15B. This applies a brake on the hydraulic motor from point W to decelerate the output shaft, as shown in FIG. 15F. At this time, however, the supply pressure Pa of the hydraulic motor in FIG. 15C becomes zero because the directional control valve is closed. However, the pressure Pb in FIG. 15D causes cavitation at point V because the forward supply has already been cut off, even though the hydraulic motor is still rotating in the forward direction due to the inertia of the vehicle. Thus, when high-speed rotation in the forward direction is decelerated and switched to the reverse direction, a problem arises such that cavitation occurs, which shortens the life of the hydraulic equipment, and a braking action cannot be effected, resulting in loss of control.
In addition, when the output shaft of the hydraulic motor is provided with an auxiliary transmission which interrupts torque using clutches, there is a problem such that, when forward-reverse switching with the shift lever, the rotational speed or the direction of the hydraulic motor before the switching does not match the rotational speed or direction of the hydraulic motor due to drag associated with coasting of the vehicle, whereby the hydraulic motor is damaged.